Make `drop_operator` function optional

runtime/examples/python-operator/op.py

@@ -1,9 +1,6 @@
 class Operator:
     def __init__(self, counter=0):
         self.counter = counter
-
-    def drop_operator(self):
-        print('drop python operator')
     
     def on_input(self, id, value, send_output):
         val_len = len(value)

runtime/src/operator/python.rs

@@ -68,9 +68,14 @@ pub fn spawn(
                 .wrap_err("on_input failed")?;
         }
 
-        operator
-            .call_method0("drop_operator")
-            .wrap_err("drop_operator failed")?;
+        if operator
+            .hasattr("drop_operator")
+            .wrap_err("failed to look for drop_operator")?
+        {
+            operator
+                .call_method0("drop_operator")
+                .wrap_err("drop_operator failed")?;
+        }
 
         Result::<_, eyre::Report>::Ok(())
     };